A technique for saving power consumption in a semiconductor circuit device has been developed in which the semiconductor circuit device includes an on-chip power supply circuit that reduces a voltage supplied from a power source external to the semiconductor circuit device and provides the reduced voltage to circuit blocks that operate functionally together (hereinafter the circuit blocks will be referred to as internal circuits) (H. Mair et. al., “A 65 nm Mobile Multimedia Applications Processor with Adaptive Power Management Scheme to Compensate for Variations,” VLSI Circuits Dig. Tech. Paper 21.5, June, 2007). By reducing the voltage to be supplied to the internal circuits from the voltage level of the external power source, power adjusted in accordance with power consumption in each individual internal circuit is supplied to the internal circuit and thereby excessive power supply is avoided. Consequently, the power consumption in the semiconductor circuit device may be reduced.
Stable voltage supply is a significant issue for power supply circuits that supplies a voltage reduced from the voltage of an external power source as described above. To solve the issue, a technique that minimizes a supply voltage drop caused by a sharp increase in power consumption in the internal circuits is being developed (refer to U.S. Pat. No. 4,952,863).
U.S. Pat. No. 4,952,863 proposes a technique in which a signal input in internal circuits is also input in a monostable multivibrator, which generates a pulse used to rapidly increase the current supply capability of power supply transistors in a power supply circuit when the voltage of the input signal reaches a certain threshold. Thus, when current consumption in the internal circuits rapidly increases, the power supply circuit may supply an amount of current commensurate with the current consumption in the internal circuits in accordance with the signal input in the internal circuits. Consequently, a drop in the reduced voltage supplied to the internal circuits is avoided.
Another technique has come into use in which a power supply circuits supply reduced voltage that varies depending on operation modes of internal circuits to the internal circuits, instead of continuously supplying a constant reduced voltage, in order to further reduce power consumption.
An issue to be solved in use of the technique is to shorten the period of time during which a reduced voltage is unstable after changing a reduced voltage. This is because the internal circuits may not be available until the reduced voltage becomes stable, which determines a delay in operation of the entire semiconductor circuit device including the internal circuits.